1. Field of the Invention
The present invention relates to a high frequency transmission line circuit, more particularly to a high frequency transmission line circuit suitable for mounting on a microwave integrated circuit (MIC).
In recent years, along with the spread of car telephones and hand-held cordless telephones, a demand has arisen for high frequency related circuits of a greater performance, low cost, and high reliability, for example, high frequency amplifying circuits. The MICs constituted as so-called printed circuits are suitable for automated production lines, are small in size, and are superior in reliability as well and have been made much use of in various fields in recent years, including the above applications.
As a method for improving the performance of high frequency amplifying circuits, the practice has been to divide the input signals for a plurality of rows of amplifiers comprised of GaAsFETs, for example, to amplify them for each row, and then combine them so as to produce the high power output.
This division and combination, however, requires a high frequency transmission line circuit split into a tree-like configuration. Such a high frequency transmission line circuit connected to the inputs of the plurality of amplifier rows is generally called a power divider, while a high frequency transmission line circuit connected to the outputs of the plurality of the amplifier rows is generally called a power combiner.
2. Description of the Related Art
As a power divider/combiner built by a high frequency transmission line circuit, there has been known the one called the Wilkinson type. This, as explained in detail later, has as one unit two parallel high frequency transmission lines and one isolation resistor. Ends, one each, of the two high frequency transmission lines are connected in common, while the other ends are independent, but are mutually connected by the isolation resistor. The actual Wilkinson type power divider/combiner has a plurality of the above units connected in tandem in a tournament configuration as shown in FIG. 1. In this case, the lengths of the high frequency transmission lines of each unit are .lambda./4, where .lambda. is the wavelength of the frequency of the high frequency signal passing through the unit.
As mentioned above, the length of the conventional high frequency transmission lines was .lambda./4. When one and two stages of such units were connected in tandem, the total length therefore became .lambda./4 and .lambda./2. In other words, in the conventional high frequency signal power divider/combiner, each of the high frequency transmission lines had a length of .lambda./4, so the distance between input-output ports in the case of a 1.fwdarw.4 division (or 4.fwdarw.1 combination) had to be .lambda./4+.lambda./4=.lambda./2 (that is, half wavelength). When considering, in particular, mounting in an MIC, this has to be improved in the sense of size reduction.
The simplest method conceivable for reducing the size would be to shorten the length .lambda./4 of the high frequency transmission lines to .lambda./8 or .lambda./16 or the like, but even if the length of the high frequency transmission lines was shortened, a practical power divider/combiner could not be realized. When the length of the high frequency transmission lines is made .lambda./4, it is easy to make the input impedance of the line and the output impedance thereof the same as the corresponding external impedances (Z.sub.S, Z.sub.L) respectively and, therefore, it is easy to achieve impedance matching between the input and output of the transmission lines.
If the length of the high frequency transmission lines were made less than .lambda./4 to reduce size by the above method, however, it would become impossible to achieve the above impedance matching.